Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic energy from the wind into mechanical energy and then subsequently converts the mechanical energy into electrical power. A horizontal-axis wind turbine includes a tower, a nacelle located at the apex of the tower, and a rotor that is supported in the nacelle. The rotor is coupled with a generator for converting the kinetic energy of the blades to electrical energy.
Traditionally, wind turbines include a main drive shaft extending from the rotor hub and into the nacelle which rotates with rotation of the rotor. The main drive shaft is operatively coupled to one or more gear stages, which may be in the form of a gear box, to produce a more suitable mechanical input to a generator also located in the nacelle. The gear box relies on various gear arrangements to provide speed and torque conversions from the rotation of the rotor and main drive shaft to the rotation of a secondary drive shaft that operates as an input to the generator. For example, the gear box may transform the relatively low rotational speed of the main drive shaft (e.g., 5 to 25 revolutions per minute (rpm)) to a relatively high rotational speed (e.g., 3,000 rpm or higher) of the secondary drive shaft which is mechanically coupled to the generator.
The generator typically includes a stator assembly and a rotor assembly concentrically disposed relative to each other within an outer housing. The stator assembly is generally fixed and stationary and includes a plurality of coils, while the rotor assembly includes a plurality of magnets and is configured to rotate relative to the stator assembly. The magnets and coils are separated from each other across a radial air gap through which the magnetic field generated by the magnets must pass. The stator assembly and rotor assembly of the generator cooperate to convert the mechanical energy received from the rotor into electrical energy so that the kinetic energy of the wind is harnessed for power generation. Specifically, the movement of the magnets of the rotor assembly past the stationary coils of the stator assembly induces an electrical current in the coils according to the precepts of Faraday's Law.
While these conventional generator designs work for their intended purpose, there has been growing interest in wind turbine drive train systems that obviate the need for gear stages. Such wind turbines are referred to as direct drive wind turbines and are characterized by having the wind turbine blades and hub mounted directly to a low-speed generator. To account for the direct drive generator's slower rotational speed, however, the diameter of the generator's rotor is often increased, for example being 5 m or more in diameter in some applications. The increased diameter increases the localized velocity of the magnets (which scales linearly with the radial position of the magnets) and further provides increased space for additional magnets and coils. Thus, while a direct drive generator rotates more slowly, the increased radial dimension of the generator provides an offset for maintaining sufficient power production. The large radial extent of direct drive generators, however, present certain design challenges for wind turbine manufacturers, especially as the size and power production of wind turbines continue to increase.
In this regard, it is generally known that the passage of the magnetic field produced by the magnets of the rotor assembly to the coils of the stator assembly depends to some degree on the width and uniformity of the air gap maintained therebetween. Thus, it is desirable and a primary design criteria for generator designers to maintain a substantially uniform (e.g., within an acceptable tolerance band) and optimized air gap between the stator and rotor assemblies. More particularly, on the one hand, the smaller the air gap, the stronger the magnetic field that interacts with the stator coils and the more current is induced therein due to passage of the magnets (i.e., the more electricity is produced). On the other hand, however, contact between the stator and rotor assemblies of the generator can do significant damage to the generator and should be avoided. Accordingly, the various forces and non-uniformities that act on the wind turbine, and ultimately the generator, must be accommodated in a manner that does not significantly disturb the air gap between the stator and rotor assemblies or allow the stator and rotor assemblies to contact each other. Thus, there are counter balancing considerations when establishing the air gap width in the generator.
From a broad perspective, these forces and non-uniformities acting on a generator may generally be divided into internal disturbances and external disturbances. Internal disturbances focus on the aspects of the generator itself that may affect the consistency of the air gap. External disturbances, on the other hand, focus on the effects that aspects of the external environment have on the air gap. Considering first internal disturbances, a primary contributor to internal disturbances is the generator's bearing arrangement. For example, the relative movement between the stator and rotor assemblies is generally established by roller element bearings. If, however, the roller elements are out of round or otherwise irregular or the bearing races are out of round or otherwise irregular, the consistency of the air gap may be negatively affected. The generator is also under the influence of magnetic loads that ultimately get transferred to the structural aspects of the wind turbine, such as the tower. These magnetic loads must be accommodated in some manner that attempts to maintain the consistency of the air gap. In conventional designs, this is typically achieved by making the structural aspects of the generator very stiff so as to suppress significant deflections under the magnetic loads imposed thereon.
While roller element bearings generally work well on a relatively small scale (i.e., the air gap may be maintained within an acceptable tolerance band), roller element bearings do not scale upwardly well. In this regard, it can become difficult to maintain acceptable tolerance values for roller element bearings larger than about 1.5 meters in diameter using, for example, conventional manufacturing techniques. In other words, it is difficult to maintain substantially perfect roundness and tolerances of the roller element bearings on a relatively large scale. As noted above, these imperfections in the roller element bearings may have a negative impact on maintaining a consistent air gap between the stator and rotor assemblies. Thus, for large scale and large load roller element bearing applications, high precision manufacturing techniques are generally required to make the designs more feasible. Of course, this increases the time, labor and cost associated with the manufacture of the generator. For direct drive wind turbine generators, which as explained above are generally large scale applications, the use of roller element bearing assemblies represents a major design challenge that may ultimately limit generator size and output.
As to external disturbances, the wind turbine rotor (i.e., the central hub and blades) is subject to a wide range of loading, including asymmetric, transient loading resulting from, for example, turbulence, wind gusts, vertical and horizontal wind shear, and other wind conditions, as well as loading from inertial and gravitational forces. In conventional drive trains having gear stages, many of these external disturbances become dampened or dissipated before reaching the generator. Thus, their impact is somewhat mitigated in conventional gear stage generator designs. In direct drive wind turbines, however, these forces ultimately get transferred to the wind turbine tower through the generator itself, thus subjecting the generator components to potential deflection and perturbations along the load path to the tower. To prevent or reduce the deflection of the generator components, and thus possible disruption of the air gap between the stator and rotor assemblies, the stator and rotor assemblies are made stiff, i.e., having significant structures associated therewith so as to withstand the forces being transmitted therethrough without significant deflection. This stiff structural requirement results in costly and heavy generator designs.
Another external aspect of some concern is the effect of thermal discursions on the consistency of the air gap. In this regard, wind turbines operate in a host of environments that experience ambient temperature changes on the order of 40-50° C. In some applications, the thermal expansions/contractions that occur due to the thermal variations may be on the order of the desired design tolerances of the generator (e.g., the tolerance band of the air gap). Thus, current designs generally provide an increased air gap width to allow for this thermal expansion/contraction of the generator components.
In addition to the above, there are additional disadvantages to current direct drive generator designs. More particularly, in view of the internal and external disturbances imposed on a typical wind turbine generator, current designs provide for an air gap width of about 5-10 mm between the stator and rotor assemblies. To enhance energy production, the magnetic flux passing through the air gap should be maximized, which suggests using permanent magnets in the rotor assembly as opposed to electromagnets, since permanent magnets generally produce stronger magnetic fields as compared to their electromagnet counterpart. Moreover, there is uncertainty whether electromagnets can generate sufficiently strong magnetic fields that can pass through air gaps on the order of 5-10 mm (again a range generally needed to accommodate the internal and external disturbances without stator/rotor contact) and result in sufficient power production. It is contemplated, for example, that much smaller air gaps would generally be required to make electromagnetic induced power production more feasible and desirable. Those smaller air gaps simply cannot be reliably obtained at this time using conventional generator designs.
While permanent magnets are attractive for direct drive generator designs, permanent magnets have certain drawbacks. For example, many permanent magnets are rare-earth permanent magnets composed of an alloy containing one or more rare earth (lanthanide) elements, such as neodymium or samarium, that are ferromagnetic metals. Representative alloys suitable for the permanent magnetic material of permanent magnets include, but are not limited to, a samarium alloy containing cobalt (SmCo5) and a neodymium alloy containing iron and boron (Nd2Fe14B). However, rare earth magnetic materials are not particularly plentiful and a significant amount of material is required for direct drive generator designs (e.g., as much as 1,000 kg of finished magnets for each mega watt (MW) of power output) due to the large size and number of magnets needed to compensate for the slower rotational speeds. These circumstances do not lend themselves to economically advantageous positions for manufacturers. Additionally, it is anticipated that rare earth magnets will become a major supply challenge for direct drive generator manufacturers.
Accordingly, there is a need for improved generator designs that address these and other disadvantages of conventional generator designs. More particularly, there is a need for a direct drive wind turbine generator design that provides enhanced control of the air gap in view of the potential internal and external disturbances imposed on the wind turbine, and wind turbine generator more particularly. Enhanced control of the air gap will provide a significantly greater number of design options that have been foreclosed in conventional generator designs.